**1. Introduction**

Modified coatings developed for various purposes, particularly coatings for metal-cutting tools, are being actively implemented in various areas of manufacturing activity. At the same time, the trends of modern manufacturing suggest toughening requirements for coatings. In particular, an increase in the cutting speed leads to an increase in temperature in the cutting zone, and, accordingly, the crucial feature of the coatings is heat resistance [1–5]. With an increase in temperature, oxidation and diffusion processes become more active and intensify tool wear [6–10]. Accordingly, one of the significant factors is the ability of a coated tool to resist the oxidation and diffusion wear. The coatings of traditional composition, such as TiN, TiC, ZrN, CrN, or (Ti,Al)N, can no longer meet the requirements of the modern manufacturing, and new coatings with enhanced properties are required. One of the ways to develop such coatings is to use a multicomponent composition and nanolayer architecture [11–15]. In particular, the introduction of such elements as Cr and Mo into the coating composition increases its resistance to heat and oxidation and diffusion effects. The coatings based on the (Ti,Al)N system are used rather widely, like nitride coatings containing Cr and Mo. However, fewer studies have considered the coatings based on the multicomponent nitrides, including a complex of these elements.

Regent and Musil [16] investigated the (Ti,Mo)N and (Ti,Cr)N coatings. The hardness of the (Ti,Mo)N coating was 38–40 GPa with the content of molybdenum (Mo) up to 10 at.%), and the δ-TiN phase (111) with miscible Mo dominated in the coating structure. The study of the (Ti,Cr)N coating detected the presence of the solid solution of (Ti,Cr)N (200) and Cr2N (111), while the phases of TiN (111), (Ti,Mo)N (200), Mo2N (111), and pure Mo were detected in the (Ti,Mo)N coating [17]. Moreover, the authors suggest that the phase of pure molybdenum in the (Ti,Mo)N coating can significantly reduce the coefficient of friction (COF) in comparison with the (Ti,Cr)N and TiN coatings [17]. The study focused on the properties of the (Ti,Cr)N coating also found that the introduction of chromium (Cr) into the composition of the TiN coating is able to significantly increase the oxidation resistance due to the formation of dense chromium oxide (Cr2O3) on the coating surface [18,19]. This oxide is characterised by high heat resistance and does not transform into chromium trioxide (CrO3) until the temperature reaches 1100 ◦C [20].

During the studies of the (Cr,Mo)N coating, the phases of fcc-CrN (111) and (200) and amorphous/nanocrystalline Mo2N were found, but no expected substitutional solid solution of (Cr,Mo)N was detected [21]. Experiments carried out by Kim et al. [22] revealed the substitutional solid solution (Cr,Mo)N with the Mo content of less than 30.4 at.%, and the maximum hardness of the coating (34 GPa) was achieved with the Mo content of 21 at.%. The studies also note that the phases formed in this coating depend on the nitrogen-to-argon ratio in a chamber. With an increase in the nitrogen content, a single fcc solid solution (Cr,Mo)N phase forms instead of the mixture of bcc hexagonal (Cr,Mo)N phases [23]. During the studies of the coating with alternating CrN/Mo2N layers, the formation of MoO3 and Cr2O3 oxides was detected upon heating to 400–500 ◦C, and it was molybdenum trioxide (MoO3) which had a crucial influence on the reduction of friction [24]. In [25–27], the investigation of the (Ti,Al,Mo)N coating revealed the presence of TiN and Mo phases (studied by the X-ray diffraction (XRD) method). A study by the XRD method detected no phase of γ-Mo2N which can be explained by the fact that in the X-ray diffraction patterns, the reflections of this phase and the phase of MoN coincide with TiN. However, the phase of γ-Mo2N was detected by the X-ray photoelectron spectroscopy (XPS) method. The hardness of the (Ti,Al,Mo)N coating reaches 40 GPa [28]. The introduction of nickel (Ni) in the composition of the (Ti,Al,Mo)N coating reduced the grain sizes from 40–50 to 10–12 nm. Meanwhile, at temperatures exceeding 500 ◦C, the Ni-containing coating wore out more intensively, and in [29], that fact was associated with the formation of the TiNiO3 oxide.

In their previous articles, authors of this paper studied the properties of the coatings with wear-resistant layers, including (Ti,Cr,Al)N [30–32], (Zr,Nb,Cr,Al)N [30,32], (Zr,Nb,Ti)N [32], (Zr,Cr,Al)N [32], (Nb,Zr,Ti,Al)N [32], and (Ti,Cr,Al,Si)N [33]. These studies show that the coatings with multi-element compositions often have better performance properties compared to binary and ternary systems.

The studies were focused on the Ti-TiN-(Ti,Cr,Mo,Al)N coating with three-layer architectures according to the recommendations described in in our past works [34,35]. It can be assumed that coating Ti-TiN-(Ti,Cr,Mo,Al)N will have good tribological properties due to the formation of MoO3 and Cr2O3 oxides with high hardness and wear resistance [18–24,29].
